The present invention relates to a process for processing a thermoplastic, enabling highly transparent articles, in particular films, to be manufactured. It also relates to special films which can be obtained by this process.
Transparent films constitute a very extensive application of thermoplastics, especially in the field of packaging. In particular, by way of example, among the polymers used for manufacturing transparent films, propylene polymers occupy an increasing place.
Improving the transparency of films, which remains a constant objective of the industries in question, may not only be achieved by optimizing the chemical nature of the polymers but also by the choice of particular processing conditions. As a general rule, the transparency of semicrystalline polymers may be increased by reducing their crystallinity and/or the size of their crystallites.
With regard to the chemical nature of the compositions used, techniques known for this purpose are in particular the addition of a comonomer so as to obtain a less crystalline random copolymer, or the addition of a nucleating agent (talc, etc.) which reduces the size of the crystallites and increases their number. With regard to the processing conditions, conventional techniques consist in particular in drawing the film in the solid state so as to break up the crystallites (for example, in the case of biaxially oriented films), or else by very rapidly cooling the films (quenching) immediately after their manufacture, so as to reduce the size of the crystallites and simultaneously to reduce the crystal-linity of the polymer. However, in the latter case, the crystallinity will tend to increase over time, and will do so all the more rapidly if the film is reheated.
Several methods have already been proposed for the purpose of improving certain mechanical and optical properties of thermoplastic-based films. Thus, in the document FR 1,211,161, such a film, manufactured beforehand, is reheated above its melting point (T.sub.f), then cooled between T.sub.f -12.degree. C. and T.sub.f, then drawn at least 150% at this temperature, and finally cooled to at least 75.degree. C. under tension.
A drawback of this process is that it requires a separate step to manufacture the film, followed by a reheating step, this being highly disadvantageous within the framework of industrial exploitation.